1. Field of the Invention
The present invention relates to a thin film forming method, a thin film forming apparatus and a solar cell and, more particularly, to a thin film forming method and apparatus for forming a thin film of a-Si or the like having an excellent thickness uniformity over a large-sized substrate.
2. Related Art
A solar cell has been noted and expected as a clean energy source, but its cost reduction is indispensable for its spread. It has, therefore, been earnestly desired to provide a thin film forming apparatus which can uniformly form an a-Si film over a large-sized substrate at a high throughput.
For forming a thin film such as an a-Si film, there has been widely used a parallel-plate (or capacitive coupling type) plasma-assisted CVD reactor. However, this reactor has a disadvantage of low throughput because each run usually processes one substrate. In contrast, the reactor to simultaneously process a plurality of substrates may become extremely large. In addition, a large-seized solar cell having desired characteristics can be hardly manufactured since the film thickness uniformity is seriously lowered with increasing the size of substrate.
A variety of investigations have been made in order to generate a uniform plasma all over the substrate and then form thin films with high thickness uniformity. However, it was found very difficult to generate uniform plasma over a large-sized substrate, which is attributed to the fundamental disadvantages of this method as will be mentioned below.
First, this method requires the precise arrangement of two plate electrodes with a prescribed distance all over the electrodes to generate uniform plasma, which is practically difficult as the substrate is larger.
Then, when the high-frequency power is fed to the electrode, the discharge takes place between the high-frequency driven electrode and grounded opposite electrode and walls of a film forming chamber. This discharge causes the self-bias potential on the electrode and to lower the uniformity of plasma density as described in Japanese Patent Laid-Open No. 7-94421.
In addition, as the electrodes are enlarged, the standing wave tends to be generated on the electrode surfaces, which also lowers the uniformity of plasma density. This plasma distribution becomes more noticeable when the VHF frequency or higher frequency is employed.
Under such a circumstance, there has been proposed another type of plasma CVD method using an inductive coupling electrode. This method is absolutely different in the mechanism for maintaining the plasma discharge from the capacitive coupling plasma CVD method; therefore, this method does not require precise arrangement of electrodes and is freed of the self-bias of the electrodes. In addition, a high-density plasma advantageous for high rate film formation can be obtained using the excitation frequency in the VHF band. The plasma CVD reactor using inductive coupling electrode is exemplified in Japanese Patent Laid-Open No. 4-236781 which employs a ladder-shaped electrode and in Japanese Patent No. 2785442 which employs a zigzagged-folded electrode.
During the investigations on a variety of inductive coupling electrodes including those having the aforementioned electrodes, the present inventors have found that as the size of inductive coupling electrodes such as the ladder-shaped or zigzag-folded electrodes is larger, the current flowing the electrode varies with the positions and the standing wave appears at unexpected positions. In short, it is difficult to create uniform plasma to cope with the large-sized substrate so far as electrode structures of the prior art are employed.
The investigation on the film uniformity using the inductive coupling electrode has been little made. On the other hand, a lot of investigations have been made to improve the film thickness uniformity in the case using the capacitive coupling electrode as described, for example, in Japanese Patent Laid-Open No. 7-94421. Here, in order to solve the problem of the distribution of plasma density caused by the self-bias potential on the electrode mentioned above, the high-frequency voltage is modulated to create intermittent discharge.
However, it is meaningless to apply the method that is effective for the capacitive coupling PCVD method to the inductive coupling PCVD because the mechanism for maintaining the discharge is absolutely different between the inductive coupling and the capacitive coupling PCVD. That is, in the capacitive coupling PCVD, the plasma discharge is maintained by the secondary electron emission from the electrodes and by the vibrations of the sheath. On the other hand, in the inductive coupling PCVD, the plasma discharge is maintained by the vibrations of the electromagnetic field which is fed from the electrodes. Therefore, the investigation results as to the capacitive coupling PCVD are not available in the inductive coupling PCVD.
Accordingly, the present inventors carried out fundamental investigations on the plasma homogenization using the inductive coupling electrodes and examined several electrode structure that positively utilizes the standing waves that caused the uniformity to deteriorate in the prior art inductive coupling electrodes. Here, a rod-shaped and a U-shaped electrode were used, which have a power feeding portion and grounding portion at the respective ends. The distance between the grounding portion and the feeding portion was set to be natural number multiple of a half wavelength of the high-frequency wave to establish the standing wave at predetermined positions over the electrode so that the predictable plasma distribution may be utilized to form the thin film with uniform thickness on the substrate.
These electrode structures apparently enables the improvement of the film thickness uniformity as compared with the prior art. However, it was also found that the plasma density varied from the feeding portion side to the grounding portion side of the electrode as the electrode is lengthened to process a large-sized substrate. The low plasma density near the grounding portion causes the film to be thin as compare with that of the feeding portion.
This phenomenon is thought to come from the fact that the high-frequency waves attenuate when propagating to the end portion of the electrode. Therefore, the present inventors have further investigated the apparatus construction and the film forming conditions to eliminate the plasma density distribution caused by the attenuation of the high-frequency power and as a result found the following facts. That is, the state of the plasma is changed by amplitude-modulating the high-frequency power in a similarly way as the capacitive coupling PCVD although the plasma maintaining mechanism is absolutely different from each other. The distribution of the plasma density is also changed by the modulation condition and these changes have repeatability.
It has been also found that in the case where a plurality of electrodes are arranged in parallel to form the thin film over a wide substrate, the film thickness distribution in the longitudinal direction of each electrode changes with the phase of the high-frequency power to be fed to electrodes.
The information is obtained for the first time by systematically investigating the film forming method using the inductive coupling electrodes, and the researches are further developed on the basis of the discoveries to clarify the relations between the feeding method and the modulation method, and the thin film distribution. Thus, the present invention has been accomplished.
An object of the present invention is to provide a thin film forming method and apparatus which make it possible to form a thin film having an excellent thickness uniformity over a large-sized substrate.
Another object of the invention is to provide a thin film forming method and apparatus which make it possible to form a thin film having excellent characteristics and thickness uniformity at high throughput.
Still another object of the invention is to fabricate a solar cell by using the aforementioned thin film forming method or apparatus and to provide a high-quality, low-cost solar cell.
According to the present invention, there is provided a thin film forming method comprising: the step of arranging an inductive coupling electrode having a power feeding portion and a grounding portion in a film forming chamber; and the step of feeding an amplitude-modulated high-frequency power to said feeding portion to generate a plasma to form a thin film over a substrate arranged to face the inductive coupling electrode.
By thus amplitude-modulating the high-frequency power to be fed to the inductive coupling electrode and by adjusting the modulation, it is possible to create the plasma having the desired density distribution and to form a thin film with a uniform thickness over a large-sized substrate. Under the various film forming conditions, the uniform plasma density distribution can be achieved by selecting the proper modulation method. For example, even under the condition for forming a high quality film at a high rate, the plasma density can be made uniform to form the thin film with uniform thickness.
Here, the xe2x80x9camplitude modulationxe2x80x9d has a meaning including the pulse modulation.
In the present invention, for example, the amplitude modulation that cuts off the high-frequency power periodically is used. Moreover, the ratio of a period for feeding the high-frequency power is preferably varied. Still moreover, the modulation frequency of the amplitude modulation is preferably varied.
The inductive coupling electrode of the invention is properly exemplified by either an electrode having a shape folded at the center or a rod-shaped electrode. It is preferred that the frequency of the high-frequency power is changed to establish standing waves between the feeding portion and the folded portion of the inductive coupling electrode or between the feeding portion and the grounding portion.
Moreover, it is preferable that the inductive coupling electrode having the center-folded shape is disposed in plurality in parallel in a common plane, and that the high-frequency powers in anti-phase are fed to the adjacent feeding portions.
By thus arranging the plurality of inductive coupling electrodes folded at the center and by shifting the phase of the high-frequency power to be fed to the adjacent feeding portions by 180 degrees, the film thickness distribution not only in the widthwise direction of the substrate but also in the longitudinal direction of the electrodes can be improved. This makes it possible to form a thin film with uniform thickness over a larger substrate. Moreover, since the distribution of the plasma density changes with the conditions of amplitude modulation, as described before, the plasma density can be further homogenized for the various film forming conditions by selecting the proper modulation conditions and by feeding anti-phase high-frequency powers to the adjacent electrodes.
According to the invention, there is provided a thin film forming apparatus comprising: a film forming chamber in which an inductive coupling electrode having a feeding portion and a grounding portion at its two ends is arranged; a high-frequency power source for feeding a high-frequency power to the feeding portion; and a waveform generator for amplitude-modulating the high-frequency power outputted from the high-frequency power source, whereby the amplitude-modulated high-frequency power is fed to the inductive coupling electrode to generate a plasma so that a thin film may be formed on a substrate arranged to face the inductive coupling electrode. Moreover, in this invention, the apparatus construction in which a plurality of electrodes having a rod shape or a center-folded shape are arranged in a common plane is preferably used.
It is preferred that the distance between the feeding portion and the grounding portion of the rod-shaped electrode or between the feeding portion and the turning portion of the center-folded shape electrode is set to be natural number multiple of a half wavelength of the high-frequency wave. It is possible to generate and maintain the plasma more stably thereby and to reproducibly form a thin film having uniform thickness.
Moreover, the inductive coupling electrode having the center-folded shape is arranged in plurality in parallel in a common plane, and there is further comprised means for controlling the phase of the high-frequency power to be fed to the feeding portions such that the high-frequency waves in anti-phase are fed to the adjacent feeding portions of the plurality of inductive coupling electrodes.
The inductive coupling electrode planes are arranged in a plurality of layers, and substrates are arranged on the two sides of each electrode layer so that thin films may be simultaneously formed on the plurality of substrates. That is, by using the inductive coupling electrodes, the so-called xe2x80x9cmulti-zone film forming methodxe2x80x9d can be adopted, without inviting the enlargement of the apparatus unlike the case of the capacitive coupling electrode. Thus, a film forming apparatus which forms thin films simultaneously on a number of substrates can be constructed. As a result, the throughput can be drastically improved to lower the cost of e.g. the solar cell.
A solar cell of the present invention is manufactured to include at least one of its thin films formed by the aforementioned thin film forming method or thin film forming apparatus of the present invention.
According to the thin film forming apparatus and method of the present invention, as has been described hereinbefore, uniform thin films with various qualities can be prepared since the conditions for forming a high quality film at a high rate can be selected. The cost for manufacturing the solar cell can be lowered while keeping the high quality. By using the multi-zone film forming method, moreover, the higher throughput can be achieved, which contributes to further cost reduction of solar cells.